To give sales reps access to global actions in Lightning for Gmail, create new or assign existing publisher layouts from the Lightning for Gmail Publisher Layouts section. Save your changes.

BELOW STEPS ARE NOT NECESSARY, BUT WILL PROVIDE A MORE TAILORED SIDE PANEL IN GMAIL: Next, your System Admin can modify user’s email application pane and add standard or custom components you want your Lightning for Gmail users to see and work with from Gmail.

From Setup, enter Lightning for Gmail in the Quick Find box, and select Lightning for Gmail Settings.